Adaptor, anchoring and scaffolding proteins play an important role in signal transduction. In many situations, the interaction between signaling proteins is mediated by small amino acid sequences binding to specific proteins domains, such as src homology (SH), pTyr-binding (PTB) or PDZ domains. These interactions are responsible for determining location, function and activity of receptors and transporter proteins among others. A few years ago, we isolated a novel protein that we named PDZK1. PDZK1, contains four PDZ protein-interactions domains and interacts with the carboxy-terminal portion of a number of membrane associated proteins including cMOAT (MRP2) the canalicular multispecific organic anion transporter associated with multidrug resistance, the cystic fibrosis transmembrane conductance regulator CFTR, the chloride channel CLC-3B, the type lla Na/Pi cotransporter, the Na-H exchanger NHE3, the chloride-anion exchanger CFEX and the high density lipoprotein (HDL) scavenger receptor SR-BI. These new findings define PDZK1 as a major player in the organization of membrane associated proteins including cell surface receptors and ion transporters. As a result, PDZK1 is likely to play an important role in biological processes as diverse as lipid metabolism and cardiovascular disease, ion channel organization and multidrug resistance. We generated a PDZK1 knockout mouse which is characterized by increased plasma cholesterol levels and markedly reduced expression of SR-BI in the liver, resulting in impaired "reverse cholesterol transport." PDZK1 joins ARM (the product of the defective gene in autosomal recessive hypercholesterolemia) in what is likely to be a growing family of cytoplasmic adaptor proteins that control the tissue specific activity of cell surface receptors. The goals of this proposal are: 1) to determine if a functional PDZK1 gene is protective against the development of atherosclerosis, 2) to study the nature of the interaction between PDZK1 and SR-BI in the liver and understand the role played by PDZK1 in optimizing reverse cholesterol transport, 3) to identify the putative molecule that plays the role of PDZK1 in the organization of SR-BI in steroidogenic organs.